1. Field of the Invention
The present invention relates to a wide-area network system for data communication, based on an ISDN (Integrated Service Digital Network), and in particular to a communication control method for use with an ISDN and a data terminal adaptor apparatus for implementing that method, whereby a substantial reduction can be achieved in unnecessary line utilization costs for the users of such a network system.
2. Description of the Related Art
FIG. 1 illustrates the general configuration of a prior art type of network system based on an ISDN. In FIG. 1, numeral 2 denotes a line of an ISDN 1, 3 denotes a data switching unit (DSU) which is connected to the line 2, 4 denotes a gateway unit which is connected to the line terminating unit 3, for changing the communication protocol, 6 denotes respective data terminals, 7 denotes a host computer, and 8 denotes respective ones of a plurality of user terminals which are connected to the host computer 7, and which can be operated by respective users to generate or display data that are to be sent or are received. The data terminal 6, the host computer 7 and the gateway unit 4 are interconnected as shown, for example by an RSC-232C interface.
The set of units 3, 4, 6, 7 8 shown in FIG. 1 will be referred to in the following as a local system. The overall wide area data network system consists of a number of such local systems which can communicate through the ISDN. That is to say, if a data terminal 6 is available in the local system of FIG. 1, a user of a user terminal 8 can transfer data to/from another user terminal within another local system that is connected to the ISDN 1, via the ISDN 1. In that case, the data terminal 6 to which the user terminal is linked by the host computer 7 will initiate accessing of a line 2 of the ISDN 1. Specifically, that data terminal 6 (which is the calling data terminal) is first supplied with necessary information specifying the user terminal which is to be called (i.e. is supplied with, and thereafter holds, the dialling number of the desired opposite user terminal). That data terminal 6 then becomes connected to the line 2 through the gateway unit 4 and DSU 5, and calls the opposite user terminal. As a result, at the other end of the line 2 the called data terminal will also become connected to that line 2, and data transfer between the two user terminals can then begin. When data transfer has been completed, the line 2 will be released, i.e. the calling data terminal 6 will initiate disconnection from the line 2, whereupon the called data terminal will also disconnect from the line 2.
With such a system, accessing and releasing of a communication channel through the ISDN are executed at the data terminals themselves.
However with such a prior art system, the problem arises that unnecessary line utilization costs will arise for the users. Specifically, due to the fact that accessing and releasing the communication channel is performed by the data terminals themselves, if a fault condition arises after a calling data terminal has accessed an ISDN communication channel (so that transfer of data between that terminal and the data terminal at the other end of the communication channel has become halted), the two data terminals will remain in a condition of accessing the ISDN communication channel, and the cost of the time for which the data terminal calling accesses the ISDN in such a condition must of course be paid by the user of the data terminal, even though no data are actually being transferred. Substantial amounts of wasteful line utilization costs may therefore occur. The same problem will occur if there is an error by the user in executing the operating procedure for a data terminal, i.e. the user of a data terminal may have to pay for line utilization costs which were incurred during periods in which no data transfer actually occurred. As data processing functions become increasingly complex and higher in level, and as the systems which are connected to the respective gateway units of an ISDN become increasingly complex, the operating procedures for executing data communication through the data terminals will accordingly become more difficult. Hence, there is an increasing probability that errors in executing the operating procedures will occur, so that there is an increased danger of unnecessary line utilization costs being incurred by the users of such a network system.
Moreover, when an ISDN communication channel has been accessed by a data terminal, and is subsequently left in the accessed condition although data communication is not actually being executed (e.g. due to some equipment failure having occurred or due to an error by the user in executing the operating procedure), in addition to the above problem of wasteful line utilization costs arising, the problem also arises that the data terminal for which such a condition occurs is no longer available for use by other users for communication through the ISDN.